Much of Optica promota was unoriginal but it was notable for two important reasons; in it Gregory proposed a method of using the transit of Venus to measure the distance of the earth to the sun, an idea later taken up and promoted by Edmond Halley and outlined an innovative design for a telescope, which he argued would not have the drawbacks of previous designs, giving an upright image, eliminating spherical aberration and minimising chromatic aberration.

Gregory’s design proposed using a secondary elliptic mirror to collect the reflection from a primary concave parabolic mirror and refocus the image back through a tiny hole in the primary mirror to a conventional eye-lens. Gregory’s design was sound but when he tried to have one commissioned he found that the instrument makers of his time were unable to make mirrors to his specification. The first Gregorian telescope was made by Robert Hooke and presented to the Royal Society in 1674.